Antenna of an electromagnetic probe for investigating geological formations

ABSTRACT

An antenna  3  of an electromagnetic probe used in investigation of geological formations GF surrounding a borehole WBH comprises a conductive base  31  and an antenna element  32 . The conductive base  31  comprises an opened non-resonant cavity  33 . The antenna element  32  is embedded in the cavity  33  and goes right through the cavity. The antenna element  32  is isolated from the conductive base  31 . The antenna element  32  is coupled to at least one electronic module via a first  34 A and a second  34 B port, respectively. The electronic module operates the antenna so as to define either a substantially pure magnetic dipole, or a substantially pure electric dipole.

FIELD OF THE INVENTION

The invention relates to an antenna of an electromagnetic probe formeasuring the electromagnetic properties of a subsurface formation in alimited zone surrounding a borehole. Another aspect of the inventionrelates to a logging tool for performing logs of subsurface formationbore hole. A particular application of the probe and the logging toolaccording to the invention relates to the oilfield services industry.

BACKGROUND OF THE INVENTION

Logging devices that measure formation electromagnetic properties (e.g.dielectric constant) are known, for example from U.S. Pat. No.3,849,721, U.S. Pat. No. 3,944,910 and U.S. Pat. No. 5,434,507. FIG. 2illustrates a logging device including a transmitter T and spacedreceivers R1, R2 mounted in a pad P that is urged against a boreholewall WBW of a well bore WB filled with drilling mud DM. Microwaveelectromagnetic energy (illustrated by dotted lines) is transmitted intothe formations, and energy that has propagated through the formations isreceived at the receiving antennas. The phase and amplitude of theenergy propagating in the formation is determined from the receiveroutput signals. The dielectric constant and the conductivity of theformations can then be obtained from the phase and amplitudemeasurements.

The transmitters and receivers comprise antennas that are assimilated tomagnetic dipoles. These dipoles are tangential to the pad face and areorientated in different directions. A broadside mode corresponds to thedipoles oriented orthogonally to the pad-axis. An endfire modecorresponds to the dipoles oriented in alignment with the pad axis. Thedepth of investigation for the broadside mode is very poor. Theinvestigation depth for the endfire mode is greater than for thebroadside mode, but the signal is usually weaker, for example at 1 GHz.The attenuation and phase-shift are measured between the two receivers.A simple inversion allows in case of a homogeneous formation to retrievethe dielectric constant and the conductivity. Typically, such a loggingdevice is unable to provide an accurate measurement of the formationproperties because of its high sensitivity to the standoff of the padrelatively to the formation or the presence of a mudcake on the boreholewall. For example, in the presence of a mudcake layer MC the number ofunknowns increase from two unknown, namely the permittivity ∈ and theconductivity σ of the formation GF (∈, σ)_(gf) to five unknowns, namelythe permittivity ∈ and the conductivity σ of the formation (∈, σ)_(gf)and of the mudcake MC (∈, σ)_(mc), and the mudcake thickness t_(mc).Consequently, accurate determination of the formation electromagneticproperties based on the attenuation and phase-shift measurements is notpossible.

The document U.S. Pat. No. 5,345,179 proposes a solution to improve thelogging device response and accuracy in the presence of a mudcake. Thelogging device comprises a plurality of cross-dipole antennas, eachbeing located in a cavity. The cross-dipole antenna houses both endfireand broadside polarizations in the same cavity.

FIGS. 3 and 4 are perspective and cross-section views schematicallyshowing a cross dipole antenna according to the prior art. Typically,such a cross dipole antenna 103 comprises two wires 132, 142 embedded ina non-resonant cavity 133 filled with a dielectric material andshort-circuited to the conductive cavity wall at one end.

FIG. 5 illustrates the current distribution for a cross dipole antennaaccording to the prior art. The current distribution J is approximatedfrom the analogy with a short-circuited transmission line. The currentdistribution on the radiating wire in the cavity can be approximated to:

J(γ)=J ₀ cos(k ₀ [γ−a])

where:

-   -   J₀ is the current amplitude,    -   a is the aperture size,    -   k₀ is the wave number in the cavity and is equal to:

${k_{0} = {\frac{\omega}{c}\sqrt{ɛ_{cavity}}}},$

-   -   ∈_(cavity) is the relative dielectric constant of the material        filling the cavity,    -   ω is the angular frequency, and    -   c is the speed of light in vacuum.

The current is maximal at the short-circuit location. This cosinusoidaland asymmetric current distribution excites a strong, parasitic electricdipole.

FIGS. 6 and 7 illustrate the electromagnetic field components Ey and Ezin the yz plane of a cross dipole antenna 103 (more precisely of theradiating wire) of the prior art, respectively.

The current flowing on the wire, for example wire 132, excites modes inthe cavity. The dominant mode is the transverse electric mode TE₁₀. Thismode contributes to a radiation pattern, which is close to a magneticpoint dipole m orthogonal to the wire. The current distribution on thewire will also excite parasitic modes, the dominant one being thetransverse magnetic mode TM₁₁. This mode corresponds to an electricdipole p normal to the aperture. These parasitic modes cause a strongasymmetry of the electromagnetic field Ey and Ez in the yz plane.

The antennas of the prior art are far from being pure magnetic dipoles.In particular, the parasitic electric dipole, normal to the apertureaffects the measurement accuracy. Further, as the mudcakeelectromagnetic properties are not determined, the inversion calculationfor determining the geological formation electromagnetic properties maynot be robust.

SUMMARY OF THE INVENTION

One goal of the invention is to propose an antenna and anelectromagnetic probe comprising at least one of such an antennaenabling measurement of the electromagnetic properties of a subsurfaceformation in a limited zone surrounding a borehole avoiding, at leastreducing the drawbacks of the prior art antennas and probes.

According to a first aspect, the invention relates to an antennacombining an antenna element having a simple mechanical design with anappropriate electronic circuit determining the behavior of the antennaeither as a substantially pure electric dipole or a substantially puremagnetic dipole.

More precisely, the first aspect of the present invention relates to anantenna of an electromagnetic probe used in investigation of geologicalformations surrounding a borehole comprising a conductive base and anantenna element, the conductive base comprising an opened non-resonantcavity, the antenna element being embedded in the cavity and going rightthrough the cavity, the antenna element being isolated from theconductive base, the antenna element being coupled to at least oneelectronic module via a first and a second port, respectively, theelectronic module operating the antenna so as to define either asubstantially pure magnetic dipole, or a substantially pure electricdipole.

Advantageously, the antenna element may be a wire strip.

The cavity may have a parallelepipedic or a cylindrical shape. Thecavity may be filled with a dielectric material.

The electronic module may comprise a first electronic module operatingthe antenna so as to define a substantially pure magnetic dipole, thefirst electronic module comprising an amplifier connected to atransformer, the transformer comprising a secondary having a centerconnected to a ground, the secondary being connected to the ports of theantenna element.

The electronic module may further comprise a second electronic moduleoperating the antenna so as to define a substantially pure electricdipole, the second electronic module comprising an amplifier, theamplifier being connected to the ports of the antenna element.

Alternatively, the electronic module may comprise an amplifier connectedto a phase-shifter, the phase-shifter being connected to a port of theantenna, the amplifier being also connected to the other port of theantenna element.

Advantageously, the amplifier is a power amplifier for an electronicmodule operating as a transmitter and a low noise amplifier for anelectronic module operating as a receiver.

Still another aspect of the invention relates to antenna modulecomprising an antenna of an electromagnetic probe according to theinvention. The conductive base may further comprise a printed circuitboard coupled to the antenna by means of the ports, the printed circuitboard comprising the at least one electronic module and a control andprocessing module.

Another aspect of the invention relates to an electromagnetic loggingapparatus used in investigation of geological formations surrounding aborehole, comprising:

-   -   a logging tool moveable through the borehole,    -   an electromagnetic probe comprising a pad mounted on the logging        device, adapted for engagement with the borehole wall by a        wall-engaging face of the pad,    -   at least one antenna mounted in the wall-engaging face and used        as a transmitting antenna,    -   a plurality of spaced antennas mounted in the wall-engaging face        and used as receiving antennas positioned in spaced relation to        the transmitting antenna,    -   a transmitter module adapted for energizing the transmitting        antenna to transmit electromagnetic energy into the formations        at a determined frequency, and    -   a receiver module adapted for receiving and processing an output        signal at each of the receiving antennas representative of        electromagnetic energy received from the formations,        wherein at least one of the receiving or transmitting antennas        is an antenna according to the invention.

A further aspect of the present invention relates to a method ofinvestigation of geological formations surrounding a borehole using anelectromagnetic logging apparatus comprising at least one transmittingantenna and at least one receiving antenna according to the invention,wherein the method comprises the steps of:

a) running the logging apparatus through the borehole and engaging a padwith a borehole wall so as to define a selected zone made of a medium tobe investigated,b) performing a first set of measurements at a deep radial depth ofinvestigation in the selected zone by:

-   -   b1) operating the antennas so that each antenna defines a        substantially pure magnetic dipole, and radiating        electromagnetic signals in the medium,    -   b2) measuring a first set of attenuation and phase shift of the        electromagnetic signals having traveled in the medium between        the transmitting and receiving antennas,        c) performing a second set of measurements at a shallow radial        depth of investigation in the selected zone by:    -   c1) operating the antennas so as each antenna defines a        substantially pure electric dipole,    -   c2) radiating electromagnetic signals into the formation        surrounding the borehole and measuring a first sub-set of        attenuation and phase shift of the electromagnetic signals        having traveled in the formation between the transmitting and        receiving antennas,    -   c3) radiating electromagnetic signals into the formation        surrounding the borehole and measuring a second sub-set of        magnitude and phase of the electromagnetic signals reflected by        the formation at a transmitting antenna input, and        d) performing an inversion calculation based on the first and        second set of measurements and determining the permittivity ∈        and the conductivity σ of the in the selected zone.

The medium may be the geological formation covered by a mudcake. Thestep d) may comprise performing an inversion calculation based on thefirst and second set of measurements and determining the permittivity ∈and the conductivity σ of the formation, the permittivity ∈, theconductivity σ and thickness t_(mc) of the mudcake.

The selected zone may comprise at least one geological feature. Thegeological feature may be a laminate, a fracture, a bed boundary or avug. The method may further comprise the steps of:

-   -   operating the transmitting antennas so that each transmitting        antenna defines a substantially pure electric dipole,    -   operating the receiving antennas so that each receiving antenna        defines a substantially pure magnetic dipole,    -   radiating electromagnetic signals in the selected zone,    -   measuring the attenuation and phase shift of the electromagnetic        signals having traveled in the formation between the        transmitting and receiving antennas, and    -   deducing the geological feature in the selected zone based on        the attenuation and phase shift.

Conversely, the transmitting antennas may be operated so that eachtransmitting antenna defines a substantially pure magnetic dipole, whilethe receiving antennas may be operated so that each receiving antennadefines a substantially pure electric dipole.

The antenna for an electromagnetic probe of the invention used ingeological surveys enables a better measurement accuracy of theformations electromagnetic properties than the antenna of theelectromagnetic propagation tool as described in the prior art. Inparticular, with the invention, it is possible to perform accuratemeasurement even if a mudcake covers the well bore wall, and whateverthe nature of the mudcake (e.g. oil-based-mud or water-based-mud).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of examples and not limitedto the accompanying figures, in which like references indicate similarelements:

FIG. 1 schematically illustrates a typical onshore hydrocarbon welllocation;

FIG. 2 schematically shows a pad according to the prior art positionedinto a borehole and contacting a mudcake formed onto the borehole wall;

FIG. 3 is a perspective view schematically showing in greater details across dipole antenna of the pad of FIG. 2 according to the prior art;

FIG. 4 is a cross-section view schematically showing in greater detailsa cross dipole antenna according to the prior art;

FIG. 5 illustrates the current distribution for a cross dipole antennaaccording to the prior art;

FIGS. 6 and 7 illustrate the electromagnetic field Ey in yz plane and Ezin yz plane of a cross dipole antenna according to the prior art,respectively;

FIG. 8 schematically shows a borehole wall contacting side view of a padfor measuring the electromagnetic properties of a subsurface formationcomprising antennas according to the invention;

FIGS. 9A and 9B are a cross-section view and a partial perspective andcross-section view showing an antenna according to a first embodiment ofthe invention, respectively;

FIGS. 10 and 11 are cross-section views schematically showing antennasaccording to a second and a third embodiment of the invention,respectively;

FIGS. 12A, 12B schematically show in greater details a first and asecond embodiment of the electronic module shown in FIGS. 8 to 11,respectively;

FIG. 13 illustrates the current distribution for an antenna according tothe invention operating into a substantially pure magnetic dipole mode;

FIGS. 14A and 14B illustrate a first embodiment of a transmitting and areceiving circuit for an antenna according to the invention operatinginto a substantially pure magnetic dipole mode, respectively;

FIG. 15 illustrates the current distribution for an antenna according tothe invention operating into a substantially pure electric dipole mode;

FIGS. 16A and 16B illustrate a first embodiment of a transmitting and areceiving circuit for an antenna according to the invention operatinginto a substantially pure electric dipole mode, respectively;

FIGS. 16C and 16D illustrate a second embodiment of a transmitting and areceiving circuit for an antenna according to the invention,respectively, the circuits being adapted to operate the antenna intoeither a substantially pure magnetic dipole mode or a substantially pureelectric dipole mode;

FIGS. 17 and 18 illustrate the electromagnetic field Ey in the yz planeand Ez in the yz plane of an antenna according to the inventionoperating into the substantially pure magnetic dipole mode,respectively;

FIGS. 19 and 20 illustrate the electromagnetic field Ey in the yz planeand Ez in the yz plane of an antenna according to the inventionoperating into the substantially pure electric dipole mode,respectively;

FIG. 21 is a table illustrating different modes of operation performedwith the antenna of the invention;

FIGS. 22 and 23 are graphics showing curves representing the reflectioncoefficient magnitude and the reflection coefficient phase with anantenna according to the invention operating into the substantially pureelectric dipole mode and a medium having constant conductivity and avarying permittivity, respectively;

FIGS. 24 and 25 are graphics showing curves representing the reflectioncoefficient magnitude and the reflection coefficient phase with anantenna according to the invention operating into the substantially puremagnetic dipole mode and a medium having constant conductivity and avarying permittivity, respectively;

FIGS. 26 and 27 are graphics showing curves representing the reflectioncoefficient magnitude and the reflection coefficient phase with anantenna according to the invention operating into the substantially pureelectric dipole mode and a medium having constant permittivity and avarying conductivity, respectively;

FIGS. 28 and 29 are graphics showing curves representing the reflectioncoefficient magnitude and the reflection coefficient phase with anantenna according to the invention operating into the substantially puremagnetic dipole mode and a medium having constant permittivity and avarying conductivity, respectively; and

FIGS. 30 and 31 are graphics showing curves representing thepermittivity ∈ and the conductivity σ of the formation following aninversion calculation based on measurements with antennas of theinvention, respectively.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically shows a typical onshore hydrocarbon well locationand surface equipments SE above a hydrocarbon geological formation GFafter drilling operations have been carried out. At this stage, i.e.before a casing string is run and before cementing operations arecarried out, the wellbore is a borehole WB filled with a fluid mixtureDM. The fluid mixture DM is typically a mixture of drilling fluid anddrilling mud. In this example, the surface equipments SE comprise an oilrig and a surface unit SU for deploying a logging tool TL in thewell-bore. The surface unit may be a vehicle coupled to the logging toolby a line LN. Further, the surface unit comprises an appropriate deviceDD for determining the depth position of the logging tool relatively tothe surface level. The logging tool TL comprises various sensors andprovides various measurement data related to the hydrocarbon geologicalformation GF and/or the fluid mixture DM. These measurement data arecollected by the logging tool TL and transmitted to the surface unit SU.The surface unit SU comprises appropriate electronic and softwarearrangements PA for processing, analyzing and storing the measurementdata provided by the logging tool TL.

The logging tool TL comprises a probe 1 for measuring theelectromagnetic properties of a subsurface formation according to theinvention. Once the logging tool is positioned at a desired depth, theprobe 1 can be deployed from the logging tool TL against the boreholewall WBW by an appropriate deploying arrangement, for example an arm.

FIG. 8 schematically shows a well-bore wall contacting face view of theprobe 1. The probe 1 comprises a pad 2. The pad is a conductive metalhousing, for example made in a metallic material like stainless steelarranged to be positioned in contact with a well-bore wall WBW. The pad2 is coupled to the tool TL by an arm (not shown). The arm enables thedeployment of the pad 2 into the borehole WB against the well-bore wallWBW.

The probe 1 further comprises transmitting and receiving antennas, forexample two transmitting antennas Tu and Td, and two receiving antennasRu and Rd. The transmitting antennas Tu and Td and the receivingantennas Ru and Rd are positioned in the pad along a line AA′ in the padface arranged to be positioned in contact with the well-bore wall WBW.The number of the transmitting and receiving antennas, and theirpositions relatively to each other, as illustrated in FIG. 8, is only anexample. The number and positions of the transmitting and receivingantennas may be different. Also, in the present description, eachantenna is either always used as a transmitting antenna or always usedas a receiving antenna. Nevertheless, an antenna is not limited to aspecific function; each antenna may be used as receiving antenna andsubsequently as transmitting antenna, or vice-versa by means of a switchfor connecting the antenna to the appropriate electronic module(transmitter module or receiver module).

The probe 1 further comprises an electronic arrangement 4 connected tothe transmitting and receiving antennas. Typically, the electronicarrangement 4 is designed such that the antenna may operate in afrequency range from around 10 MHz to around 2 GHz. The electronicarrangement 4 comprises at least one transmitter module and at least onereceiver module. Each transmitter module is arranged to excite thetransmitting antennas Tu and/or Td by applying an excitation signal.Each receiver module is arranged to determine an attenuation and a phaseshift of a reception signal provided by the receiving antenna Ru and Rdrelatively to the excitation signal.

Additionally, the electromagnetic probe 1 may comprise other type ofsensors (not shown), for example a temperature sensor, for measuringcharacteristic parameters of the fluid mixture, the mudcake, and/or theformation.

One or more coaxial cables (not shown) may be run though the arm forconnecting the electronic arrangement 4 with the tool TL. The tool TLcontains the bulk of the down-hole electronics (not shown) and providesenergy and control commands, and gathers measurements from theelectromagnetic probe 1. Alternatively, the electronic arrangement 4 maycomprise a data communication module (not shown) for directlytransmitting measurements to the surface equipment SE and receivingcontrol commands from it.

FIGS. 9A, 9B, 10 and 11 schematically show an antenna 3 according todifferent embodiments of the invention. The antenna 3 that will bedescribed hereinafter according to the different embodiments may be usedas a transmitting antenna (e.g. the transmitting antennas Tu, Td of FIG.8) or as a receiving antenna (e.g. the receiving antennas Ru, Rd of FIG.8).

In FIGS. 9A, 10 and 11, the antenna is shown fitted into the pad 2,while the pad contacts the well-bore wall WBW. Typically, as in thisexample, the medium which is investigated consists in the formation GFcovered by a mudcake MC. The mudcake MC is formed on the wellbore wallWBW by the screening of the particles of mud suspended into the fluidmixture when the fluid mixture invades the formation GF.

The antenna 3 comprises a conductive base 31 and a first antenna element32. The conductive base 31 comprises an open, non-resonant cavity 33.

The cavity 33 has a cylindrical shape. Nevertheless, the cavity 33 mayhave other shapes, for example a parallelepipedic shape (not shown). Asexample, the aperture size a defined by such a cavity may be around 10mm. Advantageously, the cavity is filled with a dielectric material. Anydielectric material may be used as the dielectric constant of saidmaterial filling the cavity has no impact on the radiation purity.

Alternatively, an element in a ferrite material (not shown) may beinserted into the cavity. The ferrite material increases the magneticdipole moment, while not changing the electric dipole moment.

The antenna element 32 goes right through the cavity while notcontacting the cavity walls. The antenna element 32 is coupled to theelectronic arrangement 4 by means of a first 34A and second 34B port.The port comprises a connection wire.

Advantageously, the antenna element 32 is coupled at the first 34A andsecond 34B port at each of its ends.

Advantageously, the antenna element 32 is positioned closed to thecavity 33 opening, while not protruding outside the cavity because thepad may be move against the well bore wall during logging operation.Preferably, in particular for measurement in reflection, the antennaelement 32 is in contact with the geological formation when the pad 2 isdeployed against the borehole wall. However, in certain application, itmay be advantageous that the cavity is closed by a cover or window (notshown) in order to retain and protect the dielectric material.Advantageously, the cover is made of a protective material, resistant toabrasion, for example PEEK (thermoplastic PolyEtherEtherKeton). However,any other material that does not perturb high-frequency-wave propagationand shows an appropriate mechanical resistance to abrasion isacceptable.

The antenna element 32 may have a strip shape. As an example, the widthof the strip is around 5 mm. The resistance against abrasion, theelectric dipole moment, and the sensitivity (in particular sensitivityin reflection in a substantially pure electrical dipole mode EDM) may beimproved by increasing the width of the strip.

In the example of FIG. 8, the antennas are oriented such that eachantenna element 32 is perpendicular to the pad axis, thus perpendicularto the bore hole axis. This corresponds to a preferred configuration inwhich the magnetic dipole is parallel to the pad axis. Thisconfiguration in a substantially pure magnetic dipole mode MDM enablesdeeper measurements in the geological formations. However, in certainapplication, it may be interesting that the antennas are oriented suchthat the antenna element is in alignment with the pad axis, thusparallel to the bore hole axis (configuration not shown).

FIGS. 9A and 9B illustrate the antenna 3 according to a firstembodiment.

The first 34A and second 34B ports pass through the conductive base 31by means of first 35A and second 35B openings. The openings 35A, 35B arepositioned into the bottom 33C of the cavity straight underneath theantenna element ends. The first 34A and second 34B ports extend into thecavity 33. Advantageously, the ports 34A, 34B are insulated relativelyto the conductive base at least when passing through the openings. As analternative, the openings 35A, 35B are filled with an insulatingmaterial in order to insulate the connection wires of the portsrelatively to the conductive base and maintain the positioning of theantenna element 32 into the cavity 33.

FIG. 10 illustrates the antenna 3 according to a second embodiment. Thefirst 34A and second 34B ports pass through the conductive base 31 bymeans of first 36A and second 36B inversed L-shaped tunnels. The tunnelsextend from the bottom of the conductive base 31 and emerge into thecavity 33 by the lateral walls 33A, 33B close to the top of the cavity.The antenna element 32 extends all along the cavity aperture. Asillustrated in FIG. 10, the antenna element 32 may also extends into aportion of the first 36A and second 36B tunnels. The first 34A andsecond 34B ports extend into the first 36A and second 36B tunnels.Advantageously, the ports 34A, 34B are insulated relatively to theconductive base 31 all along the tunnels 35A, 35B. Further, the ends ofthe antenna element 32 when extending into the portion of the first 35Aand second 35B tunnels are also insulated relatively to the conductivebase 31.

FIG. 11 illustrates the antenna 3 according to a third embodiment. Thethird embodiment differs from the first one in that the first 35A andsecond 35B openings positioned into the bottom of the cavity 33 arereplaced by a unique opening 37. The unique opening 37 is positionedsubstantially at the center of the bottom 33C of the cavity 33.Advantageously, the ports 34A, 34B are insulated relatively to theconductive base and relatively to each other at least when passingthrough the openings.

In the various embodiments, the metallic parts of the antenna may begold-plated to minimize Ohmic losses. The antenna 3 may be designedunder the form of an antenna module inserted into a slot of the pad 2.In this case, the conductive base 31 may advantageously comprise aprinted-circuit board (not shown) coupled to the antenna element 32 bymeans of the port 34A, 34B. The printed-circuit board may comprise animpedance-matching network. The impedance-matching network enablesmaximizing the power transmitted into the formation when the antenna isa transmitter, or, by reciprocity, the power received when the antennais a receiver. Advantageously, the printed circuit board and theimpedance-matching network are located closely to the antenna element inorder to improve its efficiency. For example, the printed-circuit boardmay be located at a distance inferior to a size of the cavity from theantenna element. Finally, the matching network may be designed forseveral discrete frequencies with passive components (inductances orcapacitances) or active components (variable capacitance). The activecomponents enable operating in a frequency range from 0.01 GHz to 2.0GHz with a maximized efficiency.

FIGS. 12A and 12B schematically show in greater details the electronicmodule 4 shown in FIGS. 8, 9, 10 and 11 according to a first and asecond embodiment, respectively. One of the electronic module 4 functionis to control the mode of operation of the antenna, namely to controlthat the antenna behaves like a substantially pure magnetic dipole or asubstantially pure electric dipole.

FIG. 12A schematically shows in details the electronic module 4according to the first embodiment. The various elements comprised in theelectronic module are shown in greater details in FIGS. 14A, 14B, 16Aand 16B.

The ports of the antenna element used as a transmitter T are coupled toa first 41T and a second 42T transmitter module by means of a firstswitch 40T. The first transmitter module 41T operates the antenna so asto define a substantially pure magnetic dipole.

The second transmitter module 42T operates the antenna so as to define asubstantially pure electric dipole. The ports of the antenna elementused as a receiver R are coupled to a first 41R and a second 42Rreceiver module by means of a second switch 40R. The first receivermodule 41R operates the antenna so as to define a substantially puremagnetic dipole. The second receiver module 42R operates the antenna soas to define a substantially pure electric dipole.

The switches and the various modules are coupled to a control andprocessing module 43. Depending which mode of operation is chosen, theswitch (40T or 40R) commanded by the control and processing module 43couples the antenna (transmitting T or receiving R antenna) either withthe first or with the second module (transmitter or receiver module).The calculation performed by the control and processing module 43 basedon the measurements provided by the first or second module (transmitteror receiver module) will be described hereinafter.

FIG. 12B schematically shows in details the electronic module 4according to the second embodiment. The various elements comprised inthe electronic module are shown in greater details in FIGS. 16C and 16D.

The ports of the antenna element used as a transmitter T are coupled toa unique transmitter module 44T. The transmitter module 44T operates theantenna so as to define either a substantially pure magnetic dipole or asubstantially pure electric dipole. The ports of the antenna elementused as a receiver R are coupled to a unique receiver module 44R. Thereceiver module 44R operates the antenna so as to define either asubstantially pure magnetic dipole or a substantially pure electricdipole. The transmitter 44T and the receiver 44R modules are bothcoupled to the control and processing module 43. Depending of the chosenmode of operation, the control and processing module 43 commands themode of operation of the antenna. The calculation performed by thecontrol and processing module 43 based on the measurements provided bythe unique module (transmitter or receiver module) will be describedhereinafter.

FIG. 13 illustrates the current distribution I(y) applied to the antennaelement 32 via the ports 34A, 34B, the antenna being fitted into acavity of aperture a. The current distribution is such that the currentapplied to the antenna increases from one extremity to the middle (a/2)of the antenna element, and then decreases from the middle (a/2) of theantenna element to the other extremity. As examples, the currentdistribution may be of the parabolic type, or the co-sinusoidal type,etc. . . . . With such a current distribution, the antenna operates intoa substantially pure magnetic dipole mode (MDM).

FIGS. 14A and 14B illustrate in details the transmitting 41T and thereceiving 41R module for an antenna according to the invention operatinginto a substantially pure magnetic dipole mode, respectively. Thetransmitting module 41T and the receiving module 41R are adapted to theelectronic module 4 according to the first embodiment as depicted inFIG. 12A.

FIG. 14A illustrates an example of the transmitting module 41T forapplying the hereinbefore described current distribution to the antennaelement. The transmitting module 41T comprises a power amplifier 41A anda transformer 41B. The output of the amplifier 41A is connected to aninput of the primary of the transformer 41B. The other input of theprimary of the transformer 41B is connected to a ground. The output ofthe secondary of the transformer 41B is connected to the first 34A andsecond 34B ports via the switch 40T. The center of the secondary is alsoconnected to the ground. Thus, a symmetrical voltage +V, −V is appliedto each end of the antenna shown in FIG. 13. Therefore, the currentdistribution shown in FIG. 13 may be given by:

${J(y)} = {J_{0}{\cos \left( {k_{0}\left\lbrack {y - \frac{a}{2}} \right\rbrack} \right)}}$

where:

-   -   J₀ is the current amplitude,    -   a is the aperture size,    -   k₀ is the wave number in the cavity and is equal to:

${k_{0} = {\frac{\omega}{c}\sqrt{ɛ_{cavity}}}},$

-   -   ∈_(cavity) is the relative dielectric constant of the material        filling the cavity,    -   ω is the angular frequency, and    -   c is the speed of light in vacuum.

FIG. 14B illustrates an example of the receiving module 41R. Thereceiving module 41R comprises a low noise amplifier 41C and atransformer 41D. The input of the amplifier 41C is connected to anoutput of the secondary of the transformer 41D. The other output of thesecondary of the transformer 41D is connected to a ground. The input ofthe primary of the transformer 41D is connected to the first 34A andsecond 34B ports via the switch 40R.

FIG. 15 illustrates the current distribution I(y) applied to the antennaelement 32 via the ports 34A, 34B, the antenna being fitted into acavity of aperture a. The current distribution is such that the currentapplied to the antenna increases from one extremity to the middle (a/2)of the antenna element, and then continue to increase from the middle(a/2) of the antenna element to the other extremity. As examples, thecurrent distribution may be of the linear type, or the sinusoidal type,etc. . . . . With such a current distribution, the antenna operates intoa substantially pure electric dipole mode (EDM).

FIGS. 16A and 16B illustrate in details the transmitting 42T and thereceiving 42R module for an antenna according to the invention operatinginto a substantially pure electric dipole mode, respectively. Thetransmitting module 42T and the receiving module 42R are adapted to theelectronic module 4 according to the first embodiment as depicted inFIG. 12A. The transmitting module 42T comprises a power amplifier 42A.The output of the amplifier 42A is connected to the first 34A and second34B ports via the switch 40T. Thus, the same voltage +V is applied toeach end of the antenna shown in FIG. 16.

Therefore, the current distribution shown in FIG. 15 is given by:

${J(y)} = {J_{0}{\sin \left( {k_{0}\left\lbrack {y - \frac{a}{2}} \right\rbrack} \right)}}$

FIG. 14B illustrates an example of the receiving module 42R. Thereceiving module 42R comprises a low noise amplifier 42B. The input ofthe amplifier 42B is connected to the first 34A and second 34B ports viathe switch 40R.

FIGS. 16C and 16D illustrate in details the transmitting 44T and thereceiving 44R module, the modules being adapted to operate the antennainto either a substantially pure magnetic dipole mode or a substantiallypure electric dipole mode. The transmitting module 44T and the receivingmodule 44R are adapted to the electronic module 4 according to thesecond embodiment as depicted in FIG. 12B. The second embodiment enablescontrolling the mode of operation of the antennas through the use of aphase-shifter, in a simpler manner and with less electronic elementsthan the first embodiment.

The transmitting module 44T comprises a power amplifier 45A. The outputof the amplifier 45A is connected to the first port 34A via aphase-shifter 46A. The output of the amplifier 45A is also connected tothe second port 34B. Those versed in the art understand that similarresult can be achieved when, conversely, the output of the amplifier 45Ais connected to the second port 34B via the phase-shifter 46A while theoutput of the amplifier 45A is connected to the first port 34A. Thephase shifter 46A is controlled by the control and processing module 43.When the phase-shifter 46A applies a dephasing of 180° to the signal atthe output of the amplifier, then a symmetrical voltage +V, −V isapplied to each end of the antenna. Thus, the current distribution inthe antenna shown in FIG. 13 is applied and the antenna operates into asubstantially pure electric magnetic mode (MDM). When the phase-shifter46A applies a dephasing of 0° to the signal at the output of theamplifier, then an identical voltage +V is applied to each end of theantenna. Thus, the current distribution in the antenna shown in FIG. 15is applied and the antenna operates into a substantially pure electricdipole mode (EDM).

The receiving module 44R comprises a low noise amplifier 45B. One of theports, for example the first port 34A, is connected to the input of theamplifier 45B via a phase-shifter 46B, while the second port 34B isconnected the input of the amplifier 45B. Alternatively, as indicatedabove, the connection relatively to the ports may be inversed. The phaseshifter 46B is controlled by the control and processing module 43. Whenthe phase-shifter 46A applies a dephasing of 180° to the signal at theoutput of one of the port, then the antenna operates into asubstantially pure electric magnetic mode (MDM). When the phase-shifter46A applies a dephasing of 0° to the signal at the output of one of theport, then the antenna operates into a substantially pure electricdipole mode (EDM).

FIGS. 17-20 show the electromagnetic field for an antenna according tothe first embodiment. Nevertheless, identical electromagnetic field isobserved for the antennas according to the second and third embodiment.

FIG. 17 illustrates the electromagnetic field Ey in the yz plane for anantenna operating according to the substantially pure magnetic dipolemode (MDM). FIG. 18 illustrates the electromagnetic field Ez in the yzplane of an antenna operating according to the substantially puremagnetic dipole mode (MDM).

FIG. 19 illustrates the electromagnetic field Ey in the yz plane for anantenna operating according to the substantially pure electric dipolemode (EDM). FIG. 20 illustrates the electromagnetic field Ez in the yzplane of an antenna operating according to the substantially pureelectric dipole mode (EDM).

With the antenna of the invention and contrary to the antenna of theprior art, the radiated electromagnetic field is symmetrical. There areno parasitic modes. Thus, a substantially pure magnetic dipole (FIGS. 17and 18) and a substantially pure electric dipole (FIGS. 19 and 20)orthogonal to the radiating antenna element are obtained.

The various measurements that can be performed with the antenna of theinvention will now be described.

The combination of an antenna element having a simple design coupled toan appropriate current distribution provided by an appropriateelectronic circuit enables controlling the mode of operation of theantenna, namely either as a substantially pure magnetic dipole or as asubstantially pure electric dipole.

Therefore, the antenna of the invention can replace cross-dipole antennaof the prior art. Further, as the mode of operation or type of thedipole of each antenna (transmitting antenna Tu, Td and receivingantenna Ru, Rd) of the pad can be individually selected, different typesof measurement can be performed as illustrated in FIG. 21.

A first type of measurement is the magnetic dipole mode (MDM)measurement where the transmitting Tu, Td and receiving Ru, Rd antennasof the pad are operating as substantially pure magnetic dipoles. Thisfirst type of measurement enables a deep radial depth of investigationinto the formation.

A second type of measurement is the electric dipole mode (EDM)measurement where the transmitting Tu, Td and receiving Ru, Rd antennasof the pad are operating as substantially pure electric dipoles. Thissecond type of measurement enables a shallow radial depth ofinvestigation into the mudcake when present or into the formation.

A third type of measurement is the cross-fire mode measurement (CFM)where the transmitting antennas Tu, Td of the pad are operating assubstantially pure electric dipoles and the receiving antennas Ru, Rd ofthe pad are operating as substantially pure magnetic dipoles.Reciprocally, the transmitting antennas Tu, Td of the pad are operatingas substantially pure magnetic dipoles and the receiving antennas Ru, Rdof the pad are operating as substantially pure electric dipoles.Theoretically, the signal received at the receiving antennas Ru, Rd iszero when the geological formation is homogeneous and isotropic. Thus,when the signal received at the receiving antennas Ru, Rd is not zero,this third type of measurement enables detecting dip, fractures, bedboundaries, vugs or anisotropy in the formation.

The fields and transfer impedances can be expressed as function of themedium properties. The medium may be the formation or the formationcovered by a mudcake.

Hence, in a homogeneous medium, the measured attenuation AT andphase-shift PS can be written as function of the wave number k in themedium.

For the magnetic dipole mode (MDM), the medium being the formation, itis given by:

${{AT}_{M\; D\; M} - {j\; P\; S_{M\; D\; M}}} = {{3\; {\ln \left( \frac{r_{2}}{r_{1}} \right)}} + {j\; {k\left( {r_{1} - r_{2}} \right)}} + {\ln \left( \frac{1 - {j\; {kr}_{2}}}{1 - {j\; {kr}_{1}}} \right)}}$

For the electric dipole mode (EDM), it is given by:

${{AT}_{E\; D\; M} - {j\; P\; S_{E\; D\; M}}} = {{3\; {\ln \left( \frac{r_{2}}{r_{1}} \right)}} + {j\; {k\left( {r_{1} - r_{2}} \right)}} + {\ln \left( \frac{1 - {j\; {kr}_{2}} - {k^{2}r_{2}^{2}}}{1 - {j\; {kr}_{1}} - {k^{2}r_{1}^{2}}} \right)}}$

where:

-   -   r1 and r2 are the distances between transmitters and receivers,    -   the wave number is given by:

${k = {\frac{\omega}{c}\sqrt{\mu_{r}}\sqrt{ɛ + {j\; {\sigma/\omega}\; ɛ_{0}}}}},$

-   -   ∈ is the relative medium permittivity,    -   σ is the medium conductivity, and    -   μ_(r) is the relative magnetic permeability, which is assumed to        be around 1 in logging application corresponding to the absence        of magnetic material.

The antenna of the invention has an impedance depending on the mediumdielectric properties, especially in the electric dipole mode (EDM). Inthis case, the antenna element, e.g. the strip, is preferably in contactwith the formation. Thus, it is also possible to determine the mediumdielectric properties by measuring a reflection coefficient at atransmitting antenna input. As, this measurement is limited to a shallowzone in front of the transmitting antenna, it is particularly adaptedeither for measuring the mudcake dielectric properties, or for measuringthe formation dielectric properties when no mudcake is present (e.g. inLogging While Drilling LWD application). This type of measurementenables to measure the medium dielectric properties with a fine verticalresolution, e.g. a few millimeters.

FIGS. 22-25 show curves obtained with a medium having a constantconductivity and a varying permittivity (∈ varying from 1 to 41). Inparticular, FIGS. 22 and 23 show curves representing the reflectioncoefficient magnitude and the reflection coefficient phase as a functionof the frequency with an antenna operating into the substantially pureelectric dipole mode (EDM), respectively. FIGS. 24 and 25 show curvesrepresenting the reflection coefficient magnitude and the reflectioncoefficient phase as a function of the frequency with an antennaoperating into the substantially pure magnetic dipole mode (MDM),respectively.

FIGS. 26-29 show curves obtained with a medium having a constantpermittivity and a varying conductivity (σ varying from 0 to 2 S/m). Inparticular, FIGS. 26 and 27 show curves representing the reflectioncoefficient magnitude and the reflection coefficient phase as a functionof the frequency with an antenna operating into the substantially pureelectric dipole mode (EDM), respectively. FIGS. 28 and 29 show curvesrepresenting the reflection coefficient magnitude and the reflectioncoefficient phase as a function of the frequency with an antennaoperating into the substantially pure magnetic dipole mode (MDM),respectively.

The reflection coefficient magnitude is mainly sensitive to the mediumconductivity. The reflection coefficient phase is mainly affected by themedium dielectric constant. The application of a simple inversioncalculation to the magnitude and phase measurement enables retrievingthe dielectric properties of the medium in front of the antenna, namely:

$ɛ^{*} = {{ɛ + {j\frac{\sigma}{{\omega ɛ}_{0}}\mspace{14mu} {and}\mspace{20mu} ɛ^{*}}} = {1 + {\frac{1 - \Gamma_{c}}{1 + \Gamma_{c}} \times \frac{1}{j\; \omega \; Z_{0}C_{0}}}}}$

where∈=real(∈*)σ=ω∈₀imag(∈*)

${\Gamma_{c} = \frac{\Gamma}{\Gamma_{air}}},$

Γ being the reflection coefficient and Γ_(air) being the reflectioncoefficient in the air determined by calibration before loggingoperations, andZ₀C₀ only depends on the antenna design and is determined bycalibration.

FIG. 30 shows a curve representing the permittivity ∈ of the formationfollowing an inversion calculation based on measurements with antennasof the invention. The permittivity of a first media of permittivity∈₁=12 and a second media of permittivity ∈₂=20 is represented by dottedlines. The measured permittivity ∈ is illustrated by a dotted curve.

FIG. 31 shows a curve representing the conductivity σ of the formationfollowing an inversion calculation based on measurements with antennasof the invention. The conductivity of a first media of conductivityσ₁=0.1 S/m and a second media of conductivity σ₂=1 S/m is represented bydotted lines. The measured conductivity σ is illustrated by a dottedcurve.

In both FIGS. 30 and 31, the coordinate 0 corresponds to the boundarybetween the first and second media. These curves illustrate that, withthe antenna of the invention, the resolution (i.e. the verticalresolution when the antenna is fitted in a logging tool used in avertical borehole) is around 2 mm.

The hereinbefore commented curves illustrate the good accuracy of thehereinbefore described inversion calculation based on measurementsperformed with the antenna of the invention. Further, it also enables tomeasure the medium dielectric properties with a fine verticalresolution.

The antennas of the invention are comprised in an electromagneticlogging apparatus (see FIG. 1). The electromagnetic logging apparatuscan implement a method to determine the electromagnetic properties ofthe medium surrounding the borehole. The structure and operation of suchan electromagnetic logging apparatus is described in details in thepatent application published under No EP 1 693 685 (filed on 22 Feb.2005), which is incorporated herein by reference. The electromagneticprobe of the present invention differs from the one of EP 1 693 685 inthat it comprises the antennas of the invention as hereinbeforedescribed.

While the logging apparatus is being run through the borehole and thepad engaged with the borehole wall (FIG. 1), electromagnetic signals areradiated into the formation surrounding the borehole by the transmittingantennas Tu, Td. The attenuation and phase-shift of the electromagneticsignals are measured by means of the receiving antennas Ru, Rd.

A method of investigation using the antennas of the invention will nowbe described, in a logging application where the well bore wall iscovered with a mudcake.

In a first step, the antennas are operated into the magnetic dipole modeMDM (cf. table of FIG. 21). These first set of measurements is affectedby the medium electromagnetic properties at a deep radial depth ofinvestigation, namely the formation. However, it is still affected bythe mudcake electromagnetic properties.

In a second step, the antennas are operated into the electric dipolemode EDM (cf. table of FIG. 21). These second set of measurementscomprise a first sub-set of reflection measurement and a second sub-setof transmission measurement. The second set of measurements is affectedby the medium electromagnetic properties at a shallow radial depth ofinvestigation, namely mainly the mudcake electromagnetic properties andthe mudcake thickness. The second set of measurements enables correctingthe first set of measurements for the mudcake electromagnetic propertieseffect.

With these measurements, it is now possible even in the presence of amudcake layer to determine the mudcake thickness t_(mc) the permittivity∈ and the conductivity σ of the mudcake MC (∈, σ)_(mc), and of theformation GF (∈, σ)_(gf). by means of an inversion calculation ashereinbefore described.

FINAL REMARKS

A particular application of the invention relating to a wireline toolhas been described. However, it is apparent for a person skilled in theart that the invention is also applicable to a logging-while-drillingtool. A typical logging-while-drilling tool is incorporated into abottom-hole assembly attached to the end of a drill string with a drillbit attached at the extreme end thereof. Measurements can be made eitherwhen the drill string is stationary or rotating. In the latter case anadditional measurement is made to allow the measurements to be relatedto the rotational position of the drill string in the borehole. This ispreferably done by making simultaneous measurements of the direction ofthe earth's magnetic field with a compass, which can be related to areference measurement made when the drill string is stationary.

It will also be apparent for a person skilled in the art that theinvention is applicable to onshore and offshore hydrocarbon welllocation.

It is apparent that the term “pad” used hereinbefore genericallyindicates a contacting element with the surface of the borehole wall.The particular contacting element shown in the Figures for maintainingthe antennas in engagement with the borehole wall is illustrative and itwill be apparent for a person skilled in the art that other suitablecontacting element may be implemented, for example a sonde with a backuparm, a centralizer, etc. . . . .

The same remark is also applicable to the particular probe deployingsystem shown on the Figures.

Finally, it is also apparent for a person skilled in the art thatapplication of the invention to the oilfield industry is not limited asthe invention can also be used in others types of geological surveys.

The drawings and their description illustrate rather than limit theinvention.

Any reference sign in a claim should not be construed as limiting theclaim. The word “comprising” does not exclude the presence of otherelements than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such element.

1. An antenna (3) of an electromagnetic probe used in investigation ofgeological formations (GF) surrounding a borehole (WBH) comprising aconductive base (31) and an antenna element (32), the conductive base(31) comprising an opened non-resonant cavity (33), the antenna element(32) being embedded in the cavity (33) and going right through thecavity, the antenna element (32) being isolated from the conductive base(31), the antenna element (32) being coupled to at least one electronicmodule via a first (34A) and a second (34B) port, respectively, theelectronic module operating the antenna so as to define either asubstantially pure magnetic dipole, or a substantially pure electricdipole.
 2. An antenna of an electromagnetic probe according to claim 1,wherein the antenna element is a wire strip.
 3. An antenna of anelectromagnetic probe according to claim 1 or 2, wherein the cavity (33)has a parallelepipedic or a cylindrical shape.
 4. An antenna of anelectromagnetic probe according to any one of the claims 1 to 3, whereinthe cavity (33) is filled with a dielectric material.
 5. An antenna ofan electromagnetic probe according to any one of the claims 1 to 4,wherein the electronic module comprises a first electronic module (41T,41R) operating the antenna so as to define a substantially pure magneticdipole, and wherein the first electronic module comprises an amplifier(41A, 41B) connected to a transformer (41B, 41D), the transformercomprising a secondary having a center connected to a ground, thesecondary being connected to the ports of the antenna element.
 6. Anantenna of an electromagnetic probe according to any one of the claims 1to 5, wherein the electronic module comprises a second electronic module(42T, 42R) operating the antenna so as to define a substantially pureelectric dipole, and wherein the second electronic module comprises anamplifier (42A, 42B), the amplifier being connected to the ports of theantenna element.
 7. An antenna of an electromagnetic probe according toany one of the claims 1 to 4, wherein the electronic module (44T, 44R)comprises an amplifier (45A, 45B) connected to a phase-shifter (46A,46B), the phase-shifter being connected to a port of the antenna, theamplifier being also connected to the other port of the antenna element.8. An antenna of an electromagnetic probe according to any one of theclaims 5 to 7, wherein the amplifier is a power amplifier for anelectronic module operating as a transmitter and a low noise amplifierfor an electronic module operating as a receiver.
 9. An antenna modulecomprising an antenna of an electromagnetic probe according to any oneof the claims 1 to 8, wherein the conductive base (31) further comprisesa printed circuit board (44) coupled to the antenna by means of theports, the printed circuit board (44) comprising the at least oneelectronic module and a control and processing module (43).
 10. Anantenna module according to claim 9, wherein the printed circuit board(44) further comprises an impedance-matching network and is closelylocated to the antenna element (32).
 11. An electromagnetic loggingapparatus used in investigation of geological formations (GF)surrounding a borehole (WBH), comprising: a logging tool (TL) moveablethrough the borehole, an electromagnetic probe (1) comprising a pad (2)mounted on the logging tool (TL), adapted for engagement with theborehole wall (WBW) by a wall-engaging face of the pad, at least oneantenna (Tu, Td) mounted in the wall-engaging face and used as atransmitting antenna, a plurality of spaced antennas (Ru, Rd) mounted inthe wall-engaging face and used as receiving antennas positioned inspaced relation to the transmitting antenna (Tu, Td), a transmittermodule (4T) adapted for energizing the transmitting antenna to transmitelectromagnetic energy into the formations at a determined frequency,and a receiver module (4R) adapted for receiving and processing anoutput signal at each of the receiving antennas representative ofelectromagnetic energy received from the formations, wherein at leastone of the receiving or transmitting antennas (Ru, Rd, Tu, Td) is anantenna according to any one of the claims 1 to
 8. 12. A method ofinvestigation of geological formations (GF) surrounding a borehole (WBH)using an electromagnetic logging apparatus comprising at least onetransmitting antenna (Tu, Td) and at least one receiving antenna (Ru,Rd) according to any one of the claims 1 to 8, wherein the methodcomprises the steps of: a) running the logging apparatus through theborehole and engaging a pad with a borehole wall so as to define aselected zone made of a medium to be investigated, b) performing a firstset of measurements at a deep radial depth of investigation in theselected zone by: b1) operating the antennas so that each antennadefines a substantially pure magnetic dipole (MDM), and radiatingelectromagnetic signals in the medium, b2) measuring a first set ofattenuation and phase shift of the electromagnetic signals havingtraveled in the medium between the transmitting and receiving antennas,c) performing a second set of measurements at a shallow radial depth ofinvestigation in the selected zone by: c1) operating the antennas so aseach antenna defines a substantially pure electric dipole (EDM), c2)radiating electromagnetic signals into the formation surrounding theborehole and measuring a first sub-set of attenuation and phase shift ofthe electromagnetic signals having traveled in the formation between thetransmitting and receiving antennas, c3) radiating electromagneticsignals into the formation surrounding the borehole and measuring asecond sub-set of magnitude and phase of the electromagnetic signalsreflected by the formation at a transmitting antenna input, and d)performing an inversion calculation based on the first and second set ofmeasurements and determining the permittivity ∈ and the conductivity σof the in the selected zone.
 13. A method of investigation according toclaim 12, wherein the medium comprises a geological formation covered bya mudcake, and the step d) comprises performing an inversion calculationbased on the first and second set of measurements and determining thepermittivity ∈ and the conductivity σ of the formation, the permittivity∈, the conductivity σ and thickness t_(mc) of the mudcake.
 14. A methodof investigation according to claim 12 or 13, wherein the selected zonecomprises at least one geological feature, and wherein the methodfurther comprises the steps of: operating the transmitting antennas sothat each transmitting antenna defines a substantially pure electricdipole (CFM), operating the receiving antennas so that each receivingantenna defines a substantially pure magnetic dipole (CFM), radiatingelectromagnetic signals in the selected zone, measuring the attenuationand phase shift of the electromagnetic signals having traveled in theformation between the transmitting and receiving antennas, and deducingthe geological feature in the selected zone based on the attenuation andphase shift.
 15. A method of investigation according to claim 12 or 13,wherein the selected zone comprises at least one geological feature, andwherein the method further comprises the steps of: operating thetransmitting antennas so that each transmitting antenna defines asubstantially pure magnetic dipole (CFM), operating the receivingantennas so that each receiving antenna defines a substantially pureelectric dipole (CFM), radiating electromagnetic signals in the selectedzone, measuring the attenuation and phase shift of the electromagneticsignals having traveled in the formation between the transmitting andreceiving antennas, and deducing the geological feature in the selectedzone based on the attenuation and phase shift.
 16. A method ofinvestigation according to claim 14 or 15, wherein the geologicalfeature is a laminate, a fracture, a bed boundary or a vug.